The study of electrons and holes at semiconductor interfaces has led to many interesting discoveries, particularly when both can be used as a comparison, or interact with each other as excitonic and Coulomb drag excitations. A two-dimensional electron gas (2 DEG) at an oxide heterointerface, however, is confined electronically at atomic length scales. Its properties arise from orbital-selective quantum confinement. The confinement of the 2DEG arises primarily from the self-consistent electric potential, rather than a physical quantum well thickness. As a result, its strong spatial gradients enhance its electronic correlations.
Since two-dimensional (2D) conduction at a polar/non-polar oxide heterointerface was first observed, substantial effort has been devoted to understanding the underlying physics and microscopic origin of interface conductivity. One interesting but still challenging issue is the realization of electron-hole bilayers using the 2D-confined charge carriers in oxide heterostructures.
The emergence of the 2D-confined electrons in oxide heterointerfaces is often understood within the “polar catastrophe” model. (See, Nakagawa, N., Why some interfaces cannot be sharp. Nat Mater 5, 204-209 (2006).) Applied to an interfacial electron liquid oxide heterostructure, due to the polar discontinuity at the polar/non-polar interface, an electric field in the overlying polar layer points away from the interface to the top surface. The resulting electrostatic potential diverges as the thickness of the heterostructure grows. To avoid such potential divergence, negative charge carriers accumulate at the interface, creating a so-called n-type interface. Similarly, a p-type interface can be envisaged at the interface between an oppositely oriented polar material and a non-polar material. Even though theoretical studies have predicted 2D-confined hole carriers at such interfaces, most of the experimentally-tested p-type interfaces have exhibited insulating behavior.